Re: [patch 10/21] buffer heads: Support slab defrag

Evgeniy Polyakov
2008-05-21T06:16:20Z

On Tue, May 20, 2008 at 04:28:16PM -0700, Andrew Morton (akpm@linux-foundation.org) wrote:
> It's more than efficiency. There are lots and lots of things we cannot
> do in direct-reclaim context.
>
> a) Can't lock pages (well we kinda sorta could, but generally code
> will just trylock)
>
> b) Cannot rely on the inode or the address_space being present in
> memory after we have unlocked the page.
>
> c) Cannot run iput(). Or at least, we couldn't five or six years
> ago. afaik nobody has investigated whether the situation is now
> better or worse.
>
> d) lots of deadlock scenarios - need to test __GFP_FS basically everywhere
> in which you share code with normal writeback paths.
>
> Plus e), f), g) and h). Direct-reclaim is a hostile environment.
> Things like b) are a real killer - nasty, subtle, rare,
> memory-pressure-dependent crashes.

Which basically means we can not do direct writeback at reclaim time?..

--
Evgeniy Polyakov